One common application for thermal sensors is a thermal (infrared) imaging device such as night vision equipment. One such class of thermal imaging devices includes a focal plane array of infrared detector elements or thermal sensors having pyroelectric material. The focal plane array and its associated thermal sensors are often coupled to an integrated circuit substrate with a corresponding array of contact pads and a thermal isolation structure disposed between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements or pixels of the resulting thermal image.
One type of thermal sensor includes a thermal sensitive element formed from pyroelectric material which exhibits a state of electrical polarization and/or change in dielectric constant dependent upon temperature changes of the pyroelectric material in response to incident infrared radiation. An infrared absorber and common electrode assembly are often disposed on one side of the thermal sensitive elements. A sensor signal electrode is generally disposed on the opposite side of each thermal sensitive element. The infrared absorber and common electrode assembly typically extends across the surface of the focal plane array and is attached to the thermal sensitive elements. Each thermal sensitive element generally has its own separate sensor signal electrode. Each infrared detector element or thermal sensor may be defined in part by the infrared absorber and common electrode assembly and the respective sensor signal electrode. The common electrode and the sensor signal electrode constitute capacitive plates. The pyroelectric material constitutes a dielectric or insulator disposed between the capacitive plates.
For some thermal sensors barium strontium titanate (BST) may be used to form the thermal sensitive element for the resulting thermal sensors. Various dopants may be added to the BST depending upon the desired operating characteristics for the resulting thermal sensors. The starting place for fabricating such thermal sensitive elements is typically a wafer of barium strontium titanate or other suitable pyroelectric material having a diameter of approximately one hundred (100) millimeters and an approximate thickness of three (3) millimeters. Various grinding and/or polishing processes are frequently used to reduce the thickness of the BST wafer to approximately twenty-five (25) microns or less for the finished, bonded device.